The present invention relates to a controllable light modulator which comprises at least one substrate layer with retro-reflecting elements and an addressable transmissive layer with a regular pixel structure, where at least two adjacent pixels of the pixel structure form a macro-pixel, where the spatial light modulation is controlled by a system controller.
The fields of application of spatial light modulators (SLM) are manifold and include display and projection systems for the consumer goods sector, microscopy (optical tweezers, phase filters) beam and wave front forming, optical measuring equipment (digital holography, optical sensor), and applications in maskless lithography, ultra-fast laser pulse modulation (dispersion compensation) or in terrestrial telescopes (dynamic aberration correction).
A high-quality display of images is imperative in many of those applications, e.g. for the three-dimensional representation of moving scenes in holographic displays. The values of a computed hologram of a 3D scene which are to be used to reconstruct that scene, or values of other applications which are to be written to the pixelated light modulator are typically provided in the form of a matrix of complex values. A complex value which serves to modulate both the phase and amplitude of a wave front cannot be displayed directly in a single pixel of a conventional SLM until today. However, the modulation of only one value per pixel, i.e. a phase-only or an amplitude-only modulation, only results in an insufficient holographic reconstruction of a moving 3D scene, both as regards the quantity and quality. A complete representation of the complex values can only be achieved by a complex-valued modulation if possible at the same location and at the same point of time on an SLM.
Depending on the actual type of SLM, various methods are known to achieve a simultaneous modulation of both parts of the complex values to be displayed.
For example, two separately controllable SLM can be combined and arranged very close to each other in order to simultaneously modulate both the amplitude and phase of coherent light. One SLM modulates the amplitude, the other one the phase of the incident light. Further combinations of modulation characteristics are also possible with such arrangement.
The light must first pass through one pixel of the one SLM and then through the corresponding pixel of the second SLM. This can be achieved for example in that                The first SLM is imaged onto the second SLM by a large-area optical element, e.g. a lens, or        The first SLM is imaged onto the second SLM by an array of small-sized lenses, or        The two SLMs are sandwiched together.        
These combinations of two SLMs which serve to achieve a complex-valued modulation have the disadvantage that the distance between the two SLMs is much larger than their pixel pitch, i.e. the distance between two pixels.
A typical pixel pitch of a SLM for holographic applications is between 10 μm and 50 μm. In contrast, the distance between the two SLM panels in a sandwich arrangement is several 100 μm, in arrangements where one SLM panel is imaged onto the other, their distance is even larger.
Many types of light modulators, such as liquid crystal (LC) SLMs typically have an addressable layer of liquid crystals which is embedded between transparent glass substrates. Alternatively, in a reflective display, the addressable layer is disposed between a transparent glass substrate and a reflective glass substrate.
The glass substrates typically have a thickness of between 500 μm and 700 μm.
A sandwich structure for a complex-valued modulation can be created in that a single phase-modulating SLM and a single amplitude-modulating SLM are arranged with their glass substrates one after another. When a pencil of rays which comes from the addressable layer of a pixel of the phase-modulating SLM falls on the addressable layer of a pixel of the amplitude-modulating SLM after the passage through the glass substrates, it would already be broadened at the aperture of this pixel by diffraction effects so that cross-talking of pencils of rays of adjacent pixels would occur.
When using imaging elements, there is the challenge that exactly one pixel of the first SLM must be imaged onto one pixel of the second SLM across the entire surface of the SLMs. This requires optical systems which exhibit extremely little distortion. Such requirements can hardly be fulfilled in practice. This is why cross-talking between adjacent pixels also takes place when an imaging technique is employed.
Cross-talking can be even worse if the two SLM panels, the optical imaging system or the light sources are not perfectly aligned in relation to each other.
Further, if SLM panels are combined and disposed very close to each other, such arrangements are susceptible to errors when pencils of rays are incident at an oblique angle. These pencils of rays can run from one pixel of the first SLM panel to a different, non-corresponding pixel of the second SLM panel. This cross-talking deteriorates e.g. the reconstruction quality of a holographic display because this corresponds with a wrong combination of amplitude and phase values when representing complex values by the SLMs.
In addition to the representation of complex values there are other applications where a single SLM or a single pixel of an SLM is insufficient to achieve a high-quality light modulation. Such an application is the improvement of the contrast of an amplitude-modulating SLM. An SLM pixel which does not modulate the amplitude with great perfection still transmits a certain amount of light even in the condition where the pixel is meant to be black. In contrast, the quality of the light modulation can still be improved with a sandwich-type SLM, which theoretically does not exhibit cross-talking. With the sandwich-type SLM and a combination of two SLM pixels, both functioning as amplitude-modulating pixels, almost the maximum possible brightness can be achieved if both pixels are controlled in the “white” condition. In the “black” controlled condition, the extinction is improved if the SLM pixels which serve as amplitude pixels are combined. Generally, a sandwich-type SLM therefore allows the contrast to be improved; but in practice the problem of cross-talking between pixels persists.
Another application of a sandwich-type SLM is to increase the phase modulation range: If a single SLM only allows for example a phase modulation in a range of between 0 and π, a sandwich of two equally designed phase-modulating SLMs would make it possible to extend the modulation range to 0 to 2π. Another application where sandwich-type SLMs are necessarily be used concerns the increase in the number of amplitude or phase steps. If there is for example a single phase-modulating SLM with only two displayable phase steps, 0 and π, and a second SLM which is also of a binary type but has the phase steps 0 and π/2, a sandwich of these two SLMs would make it possible to represent four phase steps, namely 0, π/2, π, 3π/2.
Further, a sandwich of more than two SLMs could make sense to increase the number of phase steps.
In the above-mentioned applications of the complex-valued SLM and sandwich-type SLM, the problem of cross-talking between the pixels persists.